Molecular-weight reduction of nitrocellulose by electron beam radiation

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the molecular-weight reduction of nitrocellulose by means of electron beam radiation.
2. Description of the Prior Art
Nitrocellulose is a product of commerce which is available with variable nitrogen content in a wide range of viscosities. It is an excellent film-forming material useful in lacquers and other types of finishes, coating for cellophane, paper, fabrics and as a component of adhesives, etc. Heretofore nitrocellulose of reduced molecular weight has been prepared by a high temperature, acidic, hydrolytic process. One such process utilizes cellulose sheets in seven steps as follows:
1. comminute cellulose sheet,
2. nitrate comminuted material,
3. stabilize nitrated material in boiling water,
4. digest slurry of nitrocellulose by passing through a digester coil at elevated temperature and pressure,
5. wash,
6. dehydrate material, and
7. compress through rolls (densificated).
It is known from British Pat. No. 830,820 that controlled depolymerization of cellulose, especially in connection with the manufacture of viscose, can be accomplished by subjecting the cellulose to an absorbed dose of ionizing radiation between 0.05 and 60 watt-seconds/gram of cellulose (0.005 to 6.0 megarads). It is known that the stability of nitrocellulose is considerably less than that of cellulose. The nitrate group of nitrocellulose readily breaks down to form NO gas which combines with oxygen to form NO.sub.2. Not only could a fire develop from such breakdown, but the NO.sub.2 could combine with water or moisture to increase acidity and in turn increase degradation of the nitrocellulose. British Pat. No. 761,051 describes the subjecting of several polymers, including cellulose nitrate, to ionizing radiation in an amount sufficient to bring about an increase in the resistance of the substance to the solvent action of organic solvents but insufficient to cause any substantial change in the mechanical properties of the substance as measured at ordinary temperatures. The amount of ionizing radiation is a small quantity which crosslinks the cellulose nitrate increasing the molecular weight and thereby decreasing solubility. Polymer degradation is not disclosed. By irradiating nitrocellulose safely with relatively large absorbed dosages it has been found surprisingly that nitrocellulose of reduced molecular weight is prepared by a process which results in improved viscosity control, increased productivity and lower energy costs. The irradiation step replaces digestion step (4) described above.
SUMMARY OF THE INVENTION
In accordance with this invention nitrocellulose of reduced molecular weight is provided by a process which comprises subjecting stabilized nitrocellulose in the presence of a damping agent to an absorbed dose of ionizing radiation in the range of about 1.0 to 50 megarads at ambient temperature.
DETAILED DESCRIPTION OF THE INVENTION
The stabilized nitrocellulose (NC) useful in this invention is available in a variety of nitrogen grades. High nitrogen nitrocellulose contains 11.8 to 12.2% by weight nitrogen; medium nitrogen nitrocellulose contains 11.3 to 11.7% by weight nitrogen; and low nitrogen nitrocellulose contains 10.9 to 11.2% by weight nitrogen. "Stabilized nitrocellulose" refers to nitrocellulose which has unstable organic sulfates removed therefrom. The stabilization is accomplished by boiling the nitrocellulose in water for at least about 2 hours. The time for stabilization will vary depending on the nitrogen content of the nitrocellulose. The nitrocellulose can be utilized in any form, e.g., sheeted fluff, flakes, diced, etc.
The nitrocellulose is wet with a suitable damping agent such as water; alcohols, e.g., ethanol, isopropanol, butanol; etc. The solids level of nitrocellulose can range from about 35 to 75% by weight solids. A preferred solids range is 60 to 75% by weight. The higher solids level permits more efficient use of electrical energy. The presence of a damping agent during irradiation is desired since the irradiation of the nitrocellulose produces free radical color formers which are absorbed by the damping agent and then removed during subsequent washing and dewatering operations. See Adolphe Chapiro, Radiation Chemistry of High Polymer, High Polymer, A Series of Monographs on the Chemistry, Physics and Technology of High Polymeric Substances, Vol. XV, John Wiley and Sons, Inc., New York, New York, 1962, page 347, Paragraph D.
The source which provides the ionizing radiation may be an electron beam accelerator capable of producing a beam of electrons having energies of 200 to 1,000 KV. With such an electron beam, relatively large quantities of energy are delivered to a small number of molecules lying in the path of the oncoming electrons. The net result of this type of energy interaction on a chemical system is both excitation and ionization with substantial molecular fragments (polymer degradation).
In addition to the radiation source disclosed in the Examples suitable sources include: Van de Graff generators; isotopic radiation, e.g., cobalt 60, cesium 137 etc. The absorbed radiation dosage levels for irradiating the nitrocellulose ranges from about 1.0 to 50 megarads. The absorbed dosage level for a particular viscosity grade of nitrocellulose is dependent on the percentage of solids and the mass thickness of the nitrocellulose. A range of 2 to 40 megarads is demonstrated in the Examples.
The irradiation is generally conducted at ambient temperature in the air. It is desirable, particularly in large scale operations, to irradiate in an inert atmosphere, e.g., nitrogen, to avoid formation of large amounts of ozone in the surrounding atmosphere.
The process of this invention is advantageous particularly since it results in more efficient energy utilization over current commercial hydrolytic procedures. In addition, narrower molecular-weight distribution and improved viscosity control are achieved.
Many different viscosity grades of nitrocellulose are produced. There are viscosity specifications for each nitrocellulose grade. With the present high temperature, acidic, hydrolytic process, it is frequently necessary to blend widely diverse nitrocellulose viscosity grades in order to obtain in-specification product. Blending, however, frequently produces nitrocellulose with broad molecular-weight distribution which can result in poor quality product, e.g., hazy nitrocellulose solutions; in the broad range of solvent systems in which nitrocellulose is used. By the process of this invention, however, the irradiation dosage level can be closely controlled and the desired viscosity level achieved. Excellent reproducibility of viscosity in duplicate runs has been achieved over a wide range of dosage levels. The improved nitrocellulose finds use as a film-forming material, coating, as a component of adhesives, etc.
EXAMPLES OF THE INVENTION
The following Examples illustrate the invention. The results of the various tests found in the Examples were obtained by the procedures indicated below.
Viscosity
a. Falling Ball Viscosity, ASTM D 301-56. This test measures the viscosity of a solution of nitrocellulose in standard solvent compositions in terms of time required for a 7.94 mm steel ball to fall through 25.4 cm of the solution in a 2.54 cm diameter tube.
b. Brookfield Viscosity is measured by means of the Brookfield LVF Viscometer. The force required to rotate a spindle in a fluid at a definite constant speed is determined. The procedure including spindle size and fluid is described in a manual from Brookfield Engineering Laboratories, Inc.
Gel Permeation Chromatography
This procedure is used to determine quantitatively the molecular weight distribution of a polymer from Gel Permeation Chromatography (G.P.C.) elution curves. The G.P.C. values are relative to polystyrene standard calibration.
MVP Stability Test
ASTM D 301-56 is used to determine whether or not nitrocellulose is properly purified. A small sample (2.5 g.) of dry nitrocellulose is heated at 134.5.degree. C. in the presence of methyl violet paper (MVP) which changes color on contact with acid fumes. The time of heating required to change the color of the paper is recorded as the degree of stability.

EXAMPLE 1
100 g. of a sample of undigested water-wet high nitrogen nitrocellulose 40% solids was irradiated at 10 and 30 megarad absorbed dosage levels to determine if irradiation would cause significant nitrate degradation or result in nitrocellulose instability. The irradiation power source was a General Electric 2 MEV Electron Resonant Transformer. The results are given in Table I. The nitrate degradation in the samples was minimal.
TABLE I__________________________________________________________________________ Acidity Analyzed After Alcohol Dehydration Absorbed Irradia- MVP Dosage tion % Nitrogen Falling Ball Viscosity* Acidity StabilitySample (Megarads) (% as H.sub.2 SO.sub.4) (Nitrometer) (8.38 mm Steel Ball Secs.) (% as H.sub.2 SO.sub. (minutes)__________________________________________________________________________Control 0 0.03 12.26 79.9 0.03 19A 10 0.09 12.19 30.1 0.06 19B 30 0.23 12.28 3.9 0.09 13__________________________________________________________________________ *According to ASTM Method D301-56.
EXAMPLE 2
Approximately 200.0 g. samples of water-wet high nitrogen undigested nitrocellulose were vacuum filtered in the laboratory to a solids content of 42%. The bulk density of the product was 0.2 g./cc. The samples were irradiated by means of the radiation source described in Example 1 over an absorbed dosage level range of 2 to 40 megarads. The results are set forth in Table II.
Eight of the ten duplicate radiation runs show excellent viscosity checks. There were no problems with excessive nitrate degradation as evidenced by the nitrogen analysis and the MVP stability tests. The irradiated samples showed improved molecular-weight uniformity [polydispersity (d)] with increased radiation dosage level.
TABLE II__________________________________________________________________________Radiation Conditions MVPExposure Absorbed Viscosity Data Gel Permeation Chromatography Nitrogen Stability Time Current Dosage Falling Ball Visc. Brookfield (%) TestSample (Seconds) (Milliamps) (Megarads) (7.94 mm S.B. Secs.) (cps.) Mw Mn d (Nitrometer) (Minutes)__________________________________________________________________________1* 0 0 0 >1000 >100,000 727,300 69,121 10.5 12.54 92* 0 0 0 >1000 >100,000 743,680 62,717 11.9 12.37 9A-1 21.6 1 2 127.9 55,500 433,220 56,431 7.7 12.46 10A-2 21.6 1 2 121.0 58,500 396,630 57,715 6.9 12.39 10B-1 43.2 1 4 32.2 17,500 290,500 53,071 5.5 12.38 19B-2 43.2 1 4 32.6 16,500 310,180 56,397 5.5 12.41 16C-1 64.8 1 6 11.3 5,750 292,770 58,503 5.0 12.35 11C-2 64.8 1 6 6.7 2,700 319,350 60,455 5.3 12.45 11D-1 108 1 10 3.5 1,338 188,270 41,608 4.5 12.43 15D-2 108 1 10 2.5 975 179,220 46,400 3.9 12.43 15E-1 162 1 15 1.0 390 126,090 31,819 4.0 12.37 11E-2 162 1 15 1.1 400 121,770 32,705 3.7 12.32 14F-1 216 1 20 0.4 160 113,930 29,134 3.9 12.38 15F-2 216 1 20 0.4 180 125,660 31,246 4.0 12.43 18G-1 270 1 25 0.25 85 77,511 22,747 3.4 12.31 15G-2 270 1 25 0.28 85 82,174 24,473 3.4 12.25 19H-1 324 1 30 0.208 48 69,296 20,403 3.4 12.33 10H-2 324 1 30 0.201 42 65,750 20,468 3.2 12.35 10I-1 378 1 35 0.184 30 49,715 16,146 3.1 12.20 15I-2 378 1 35 0.158 24.5 51,587 16,146 3.2 12.18 13J-1 432 1 40 0.13 21.5 42,983 14,674 2.9 12.15 11J-2 432 1 40 0.134 28.5 45,689 15,007 3.0 12.19 11__________________________________________________________________________ *Control
EXAMPLE 3
Previous irradiations were made using only high nitrogen (11.8-12.2% nitrogen), undigested nitrocellulose. The following set of data also includes medium nitrogen (11.3-11.7% nitrogen) and low nitrogen (10.9-11.2% nitrogen) nitrocellulose. In addition, all nitrogen grades of nitrocellulose were irradiated at a higher solids level (.about.60%) to obtain improved energy utilization. The percent solids and bulk density of the three nitrogen levels of nitrocellulose are as follows:
______________________________________Nitrocellulose Solids Bulk Density(Nitrogen Level).sup.1 (%) (g./cc.)______________________________________10.9-11.2 57.2 0.16011.3-11.7 64.7 0.16811.8-12.2 57.0 0.160______________________________________ .sup.1 Industrial NC is usually nitrated to a higher nitrogen level (+0.1 to 0.2% nitrogen) to allow for some nitrate degradation which occurs during the hydrolytic molecular-weight reduction step.
The procedure of Example 1 was repeated, the data and results being set forth in Table III. The high nitrogen irradiation nitrocellulose samples show a higher solids level of nitrocellulose (60 vs. 42% as shown in Example 2) does give lower viscosities at comparable dosage levels. This confirms the improved energy utilization at higher nitrocellulose solids. The data in Table III also show that the lower nitrogen levels of nitrocellulose require higher radiation dosages to obtain comparable viscosity levels. In all cases where duplicate runs were made, the reproducibility of viscosity data was very good. The irradiated nitrocellulose samples generally showed improved molecular-weight distribution [polydispersity (d)] with increased radiation dosage level.
TABLE III__________________________________________________________________________ Viscosity Data Radiation Conditions % Acidity Falling Ball MVP Exposure Current Absorbed as H.sub.2 SO.sub.4 Viscosity Brook- Gel Permeation Nitrogen Stability Time (Milli- Dosage After (7.94 mm field Chromatography (%) TestSample (Secs.) amps) (Megarads) Radiation S.B. Secs.) (cps.) Mw Mn d trometer) (Minutes)__________________________________________________________________________Low NitrogenNitrocelluloseControl** 0 0 0 -- 147.3 45,000 503,470 20,390 24.7 11.14 15A-1 35.6 1 3.3 0.07 67.7 28,750 376,770 116,570 3.2 11.16 12A-2 35.6 1 3.3 0.12 62.0 26,000 351,770 103,270 3.0 11.12 11B-1 86.4 1 8 0.20 5.2 2,800 245,550 64,626 3.8 11.05 11B-2 86.4 1 8 0.28 5.2 2,900 215,910 51,941 4.2 10.95 11C-1 216 1 20 0.30 0.65 239 124,490 23,872 5.2 11.00 9C-2 216 1 20 0.30 0.58 213 112,760 22,221 5.1 10.99 9D-1 291.6 1 27 0.20 0.27 99 95,616 21,610 4.4 10.88 9D-2 291.6 1 27 0.20 0.27 101 95,791 23,351 4.1 11.04 9E-1 378 1 35 0.17 0.14 53 66,109 18,227 3.6 10.78 8E-2 378 1 35 0.20 0.11 40 67,691 19,427 3.5 10.76 8MediumNitrogenNitrocelluloseControl** 0 0 0 -- 77.3 39,500 907,290 66,263 13.7 11.72 16A-1 89.6 1 8.3 0.12 3.3 1,300 265,990 47,093 5.6 11.47 11B-1 89.6 1 8.3 0.15 3.6 1,400 238,460 46,181 5.2 11.42 12A-2 216 1 20 0.30 0.46 169 132,341 30,104 4.4 11.35 23B-2 216 1 20 0.33 0.39 144 126,410 29,648 4.3 11.32 23High NitrogenNitrocelluloseControl** 0 0 0 -- 118.5 58,500 1,038,800 76,454 13.6 * 7A 21.6 1 2 0.28 77.3 28,500 369,790 56,776 6.5 * 7B 43.2 1 4 0.25 18.0 7,650 255,210 62,516 4.1 * 7C 64.8 1 6 0.20 6.8 2,780 222,440 45,674 4.9 * 7D-1 89.6 1 8.3 0.08 2.7 1,250 212,530 39,817 5.3 * 6D-2 89.6 1 8.3 0.12 2.9 1,760 237,520 47,031 5.1 * 5E 108 1 10.0 0.20 1.57 650 135,970 35,472 3.8 * 7F-1 162 1 15.0 0.17 0.57 194 124,360 34,357 3.6 * 7F-2 162 1 15.0 0.17 0.54 210 117,380 29,501 3.9 * 7G-1 216 1 20.0 0.18 0.31 112 99,138 23,822 4.2 * 5G-2 216 1 20.0 0.27 0.22 82 91,558 26,258 3.5 * 5H-1 270 1 25.0 0.30 0.18 67 83,354 20,456 3.8 * 6H-2 270 1 25.0 0.32 0.16 58 84,313 22,632 3.7 * 6I-1 378 1 35.0 0.20 0.048 17.6 53,204 14,123 3.8 * 6I-2 378 1 35.0 0.20 0.043 15.9 -- -- -- * 6J-1 432 1 40.0 0.18 0.029 10.7 -- -- -- * --J-2 432 1 40.0 0.20 0.031 11.2 62,141 17,510 3.5 11.91 15__________________________________________________________________________ *Samples were prepared in the dry state to facilitate analysis. The samples were stored for 6 weeks in this state. Prolonged storage of nitrocellulose in the dry state accelerates nitrate degradation; hence, the samples did not meet the minimum stability requirements (8 minutes) for determining nitrogen analysis. **Viscosity data for control samples were based on 8% solids in standard solvent system. All other viscosity data are based on 12.2% solids.
EXAMPLE 4
Industrial Termeered (centrifuged) nitrocellulose generally has a solids level ranging from 65 to 72%. To further improve energy utilization, the following nitrocellulose samples (170.0 g.) were centrifuged and oven dried at 60.degree. C. to approximate a solids levels range of 65 to 75%.
______________________________________Nitrocellulose Solids Bulk Density(Nitrogen Level) (%) (g./cc.)______________________________________10.9-11.2 65.3 0.17611.3-11.7 71.6 0.15911.8-12.2 68.7 0.15211.8-12.2 74.9 0.153______________________________________
The procedure of Example 1 was repeated, the data and results being set forth in Table IV.
The nitrocellulose radiation data reported in Example 4 shows further improvement in energy utilization as the nitrocellulose solids is increased.
TABLE IV__________________________________________________________________________ Viscosity Data Radiation Conditions % Acidity Falling Ball MVP Exposure Current Absorbed as H.sub.2 SO.sub.4 Viscosity Brook- Gel Permeation Nitrogen Stability Time (Milli- Dosage After (7.94 mm field Chromatography (%) TestSample (Secs.) amps) (Megarads) Radiation S.B. Secs.) (cps.) Mw Mn d trometer) (Minutes)__________________________________________________________________________Low NitrogenNitrocelluloseControl 0 0 0 -- 15.2 7,420 523,580 60,553 8.6 11.00 14A 38.9 1 3.6 0.21 9.5 4,700 281,850 48,265 5.8 10.94 37B 82.1 1 7.6 0.37 2.7 1,300 165,310 27,944 5.9 10.89 33C 108.0 1 10.0 0.46 1.3 630 139,870 31,157 4.5 10.93 23D 162.0 1 15.0 0.61 0.57 238 107,260 21,242 5.0 10.75 28E 216.0 1 20.0 1.05 0.37 136 85,378 20,174 4.2 10.43 17F-1 270.0 1 25.0 1.15 0.16 59 71,296 19,331 3.7 10.54 10F-2 270.0 1 25.0 1.32 0.18 66 66,510 17,923 3.7 10.40 10G 324.0 1 30.0 1.62 0.14 51 53,949 14,464 3.7 -- 13H-1 367.2 1 34.0 1.69 0.10 37 46,708 14,812 3.1 10.64 14H-2 367.2 1 34.0 2.40 0.13 48 51,117 14,373 3.6 10.66 8I-1 432.0 1 40.0 1.88 0.057 21 43,956 9,738 4.5 10.75 17I-2 432.0 1 40.0 1.99 0.067 24.5 43,380 11,568 3.7 10.38 14MediumNitrogenNitrocelluloseControl 0 0 0 -- 76.8 18,200 726,930 65,282 11.1 11.60 13A 32.4 1 3.0 0.15 18.4 8,260 228,460 41,826 5.5 11.50 28A-1 54.0 1 5.0 0.24 6.8 3,060 176,890 30,908 5.7 11.45 35A-2 54.0 1 5.0 0.21 7.7 3,288 158,020 24,720 6.4 11.51 12B-1 81.0 1 7.5 0.39 2.5 1,108 152,950 33,692 4.5 11.44 35B-2 81.0 1 7.5 0.33 2.4 1,113 146,040 33,316 4.4 11.45 35C-1 108.0 1 10.0 0.48 1.35 585 132,660 29,710 4.5 11.46 28C-2 108.0 1 10.0 1.43 1.30 565 137,370 25,374 5.4 11.40 28D 162.0 1 15.0 0.74 0.43 163 104,090 22,393 4.6 11.40 22E-1 189.0 1 17.5 0.84 0.34 125 88,607 18,890 4.7 11.34 20E-2 189.0 1 17.5 0.81 0.32 118 69,737 21,434 3.3 11.33 19F-1 216 1 20.0 1.00 0.22 81 83,006 18,776 4.4 11.41 21F-2 216 1 20.0 0.89 0.21 77 86,144 19,687 4.4 11.35 20G-1 237.6 1 22.0 0.82 0.16 59 81,863 23,181 3.5 11.37 20G-2 237.6 1 22.0 2.90 0.16 59 76,856 20,015 3.8 11.29 19H 270.0 1 25.0 1.07 0.11 41 68,610 16,875 4.1 11.29 22High NitrogenNitrocelluloseControl 0 0 0 -- 1500+ >100,000 954,470 108,240 8.8 12.51 9A-1 5.4 1 0.5 0.20 800+ >100,000 587,970 65,800 8.9 12.40 13A-2 5.4 1 0.5 0.21 800+ >100,000 628,090 62,046 10.1 12.36 12B-1 13.0 1 1.2 0.27 148.6 72,300 455,680 73,302 6.2 -- 10B-2 13.0 1 1.2 0.25 154 69,700 452,330 45,922 9.8 12.38 10C 17.3 1 1.6 0.28 95.1 47,000 463,730 78,072 5.9 -- 13D 21.6 1 2.0 0.30 73.7 33,900 434,990 57,914 7.7 -- 10E-1 32.4 1 3.0 0.38 26.3 14,560 377,990 49,265 7.7 12.30 12E-2 32.4 1 3.0 0.34 29.5 14,560 379,930 42,602 8.9 12.30 9F 43.2 1 4.0 0.44 14.8 7,400 390,720 70,119 5.6 -- 12G-1 68.0 1 6.3 0.52 5.1 2,528 214,480 38,286 5.6 12.43 12G-2 68.0 1 6.3 0.48 5.4 2,760 216,250 40,994 5.3 -- 14H 108.0 1 10.0 0.86 1.1 456 145,330 36,283 4.1 12.35 13I 162.0 1 15.0 0.70 0.76 279 111,060 28,148 3.9 12.39 13J-1 205.2 1 19.0 1.16 0.26 96 91,302 26,077 3.5 12.36 8J-2 205.2 1 19.0 1.09 0.24 88 85,525 27,011 3.9 12.11 12K-1 280.8 1 26.0 1.54 0.16 59 64,097 17,874 3.6 -- 7K-2 280.8 1 26.0 1.48 0.14 52 63,347 18,300 3.5 -- 6L 302.4 1 28.0 1.64 0.125 46 60,783 18,071 3.4 12.24 10M-1 324.0 1 30.0 1.80 0.059 21.7 52,075 16,565 3.1 12.26 15M-2 324.0 1 30.0 1.63 0.062 22.8 53,430 16,295 3.3 12.25 10N-1 356.4 1 33.0 2.28 0.044 16.3 47,864 15,022 3.3 -- 6N-2 356.4 1 33.0 1.93 0.049 19.2 51,857 16,173 3.2 12.18 15__________________________________________________________________________

Number	Date	Country
245,916	Jul 1963	AU
830,820	Mar 1960	UK

Molecular-weight reduction of nitrocellulose by electron beam radiation

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